Diesel engines used onboard certain types of vessels and/or used in certain stationary applications are typically of the well known two-stroke type or of the well known four-stroke type.
Two-stroke cross-head engines used in marine and/or stationary applications are equipped with two separate lubricating oil systems. One lubricating system comprises so-called system oil/system lubricant that normally is used for lubrication and cooling of the engine's bearings and e.g. oil-cooled pistons as well as for activation and/or control of various valves or the like. The other lubricating system comprises an all-loss lubricant (cylinder oil/cylinder lubricant) that normally is used for lubrication of the engine's cylinders, piston rings and piston skirt.
In typical two-stroke cross-head engines, the cylinder oil is spent continuously by each turn of the engine whereas the system oil in principle is not spent (except by smaller unintentional leakages). The lubrication system comprising the cylinder oil is also often referred to as an “all-loss” lubrication system as the oil is spent. The use of and various types of both system oil(s) and cylinder oil(s) is very well known in the art.
Typical cylinder oils usually have an SAE (Society of Automotive Engineering) viscosity equivalent to about 50 and normally have a total base number (BN) of about 40 to 70 for the neutralisation of acid products produced during the combustion process. Typical system oils usually have an SAE viscosity of about 30 with a relatively low BN content, typically below 10. These exemplary values may vary dependent on the actual application and the specific design of the systems that the oils are used in.
In recent two-stroke cross-head engine designs involving electronic and/or hydraulic control and/or activation of valves, etc., the minimum performance requirements of the system oil has been substantially increased compared to earlier design using traditional mechanical control/activation.
Four-stroke, trunk piston (diesel) engines, however, typically use only a single oil type for lubrication and cooling. Such engines are typically used as secondary/auxiliary or propulsion engines on ships, or in stationary power generation or liquid/gas transmission applications. Such used oils typically have a SAE viscosity of about 30 or 40. While the system oil of two-stroke cross-head engines typically remains within its specified performance limits for an extended period of time, trunk piston engine oils are constantly affected by exposure to the combustion process.
The performance level of lubricants is typically measured periodically and may not go beyond certain limits if the oiled component's condition should not be jeopardized.
Another characteristic of diesel engines is that their mechanical efficiency is reduced significantly for reduced engine power or speed or output (forth only referred to as engine power). This results in an increased specific fuel consumption of the particular engine when the engine is operating at reduced power. An additional characteristic is that the stress of the bearings in the engine is reduced for reduced engine power. Further, the thickness of the oil film on the engine parts is increased at reduced engine power compared to the thickness at full/maximum engine output thereby reducing the load of the engine components. This holds for all types of bearings that is a part of the engine's crank mechanism when the temperature of the lubricating oils is kept substantially constant regardless of the load of the engine, which always is aimed for, by suitable adjustment of the amount of lubricating oil that is fed through the related oil coolers usually using water as a coolant.
The actual power of an engine implicitly defines the actual pressure condition in the engine's cylinders, i.e. maximal pressure, compression pressure and mean pressure, which are the main determining factors for the stress or strain on the bearings. The number of revolutions is also a known function of the load of the engine for engines that are directly coupled to a propeller.
Additionally, the stress or strain of the bearings will at any time be influenced by the specific viscosity of the system oil, which may ongoing be influenced e.g. by pollution of the oil during operation of the engine.
A diesel engine's frictional loss is mainly of a viscose character. An increase in the viscosity of the system oil will therefore result in a diminished efficiency, increased fuel consumption and increased emissions.